The glutathione S-transferases (GST) are a ubiquitous family of enzymes with dual substrate specificities that perform important biochemical functions including xenobiotic biotransformation and detoxification, drug metabolism, and protection of tissues against peroxidative damage and subsequent inflammatory responses. The basic reaction catalyzed by these enzymes is the conjugation of an electrophilic substrate with reduced glutathione (GSH), which results in either activation or deactivation/detoxification of the substrate. The requirement for conjugating reduced GSH to a wide variety of substrates necessitates a diversity in GST structures in various organisms and cell types.
GSTs are homodimeric or heterodimeric proteins localized in the cell cytosol. The major isozymes share common structural and catalytic properties, and in man have been classified into four major classes, Alpha, Mu, Pi, and Theta. The two largest classes, Alpha and Mu, are identified by their respective protein isoelectric points: pI.about.7.5-9.0 (Alpha) and pI.about.6.6 (Mu). Each GST possesses a common binding site for GSH and a variable hydrophobic binding site. The hydrophobic binding site in each isozyme is specific for particular electrophilic substrates.
In most cases, GSTs perform the beneficial function of deactivating and detoxifying potentially mutagenic and carcinogenic chemicals. However, in some cases their action is detrimental and produces mutagenic and carcinogenic compounds. Some forms of rat and human GSTs are thus reliable preneoplastic markers. Expression of human GSTs in bacterial strains, such as Salmonella typhimurium, used in the well known Ames test for mutagenicity, has helped to establish the role of these enzymes in mutagenesis. Studies have shown that dihalomethanes are more mutagenic in bacterial cells which express human GST than in cells which do not express GST (Thier, R. et al. (1993) Proc. Natl. Acad. Sci. 90: 8576-8580). The mutagenicity of ethylene dibromide and ethylene dichloride is increased in bacterial cells expressing the human Alpha GST, A1-1, while the mutagenicity of aflatoxin B1 is substantially reduced by enhancing the expression of GST (Simula, T. P. et al. (1993) Carcinogenesis 14: 1371-6).
GST has been implicated in the acquired resistance of many cancers to drug treatment. Multi-drug resistance occurs when cancer cells are treated with and subsequently become resistant to cytotoxic drugs. In some drug resistant cancers, elevated GST levels are observed. It is believed that the drug being used to treat the cancer is deactivated by the GST-catalyzed GSH conjugation reaction. The increased GST levels also protect the cancer cells from other cytotoxic agents for which that GST has affinity. Increased levels of A1-1 in tumors has been linked to drug resistance induced by cyclophosphamide treatment (Dirven H. A. et al. (1994) Cancer Res. 54: 6215-20).
Ovarian cancer cell lines resistant to adriamycin, cisplatin, and various alkylating agents such as nitrosourea and cyclophosphamide have increased levels of GSH. Chinese hamster ovary (CHO) cell lines resistant to cisplatin have increasing levels of GST. The introduction of GST cDNA makes the recipient CHO cells 1.4-3.0 fold more resistant to cisplatin. Another example of GST involvement in cell proliferation is the decreased expression of GST mRNA in human cancer cell lines resistant to buthionine solfoximine (BSO). BSO is a synthetic amino acid that irreversibly inhibits glutathione biosynthesis. Nuclear run-on assay showed that the transcriptional activity of GST is decreased in BSO-resistant cells, and transient transfection of GST promoter-chloramphenical acetyltransferase constructs revealed that the sequence between -130 and -80 base pairs of the 5'-flanking region are at least partially responsible for the decreased expression of the GST gene (Yokomizo A. et al. (1995) J. Biol. Chem. 270 (33): 19451-19457).
The discovery of a new human glutathione-S-transferase and the polynucleotides encoding it satisfies a need in the art by providing new compositions which are useful in the diagnosis, prevention, and treatment of diseases associated with cell proliferation, in particular, cancers and immune response.